This invention is directed to an improvement in a method for forming articles from thermoplastics in the solid state.
Hollow containers, such as cans, can be produced by a method in which the periphery of a billet--also referred to as "blank" or "preform"--of thermoplastic material which is orientable in the solid phase is clamped and the central, unclamped portion is gradually deformed while in the solid state by forcing a suitably shaped plunger down on it, resulting in plastic being drawn from under the plunger to form the sidewalls of the container as the plunger descends. The sidewalls of the resulting container are oriented in the direction of the draw. This method will herein be referred to as "stretch-forming". Stretch-forming methods and apparatus are described, i.a., in U.S. Pat. Nos. 3,499,188, 3,546,746 and 3,757,718 to Johnson and U.S. Pat. No. 3,606,958 to Coffman.
It has been found in such stretch-forming of predominantly crystalline thermoplastics in the solid phase that in depths of draw.sup.(1) up to about 1.25:1 there is a relatively constant ratio between the thickness of the original billet and the thickness of the sidewalls of the drawn article, regardless of the depth of draw. This observed thickness ratio is characteristic for each thermoplastic and corresponds approximately to the so-called "natural draw ratio".sup.(2) of the material. In the case of polypropylene, for example, this ratio is about 6:1. FNT (1) Depth of draw refers to the ratio between the diameter or average length of side of the billet and the length of the drawing stroke in which the sidewalls are formed. FNT (2) The term "draw ratio" is ordinarily applied in the drawing of filaments and refers to the ratio of the final linear dimension after drawing to the original linear dimension. In the drawing of filaments, it is the ratio of denier of the filament after stretching to denier before stretching. Crystalline, substantially unoriented fibers, upon drawing in the solid state usually show a shoulder effect; the undrawn and drawn portions of such fibers are separated by a shoulder portion at which the fiber "necks down". The "natural draw ratio" is that draw ratio at which the entire fiber has been stretched to the reduced diameter which it naturally tends to assume. More generally expressed, the natural draw ratio is the draw ratio at the point in a stress-strain diagram where the tensile stress begins to rise sharply after the second or lower yield point, as defined in ASTM standard on Plastics, Appendix II, D628.
It is a consequence of this effect that articles having bottom and sidewalls of approximately equal thickness are formed by stretch-forming at a depth of draw of about 1.25:1. This happens to be a desired depth of draw for many articles. The method is therefore useful for forming many typical articles, such as cans and deep boxes.
Another consequence of the described effect is that it is not possible to produce, by stretch-forming, articles of substantial draw depth which have an average wall thickness substantially greater than about 1/5 or 1/6 of original billet thickness.
Another method of forming hollow containers is the method that has become known as "solid state pressure forming". This is described, for example, in U.K. Patent No. 1,367,338. A variant of the pressure-forming method, adapted to form non-circular articles, is described in U.S. Pat. No. 3,859,028.
The pressure-forming method comprises introducing a blank or sheet of thermoplastic material in the solid state between a forming plug and a mold having an end opening, clamping it into place, moving the forming plug into the mold opening with the blank to carry a bulged portion of the blank into the mold, and introducing pressure fluid into the bulged portion to form the article in the mold.
The solid phase pressure forming method is distinguished from stretch forming by the shape of the plug, which has a rounded bottom, so as to push the plastic blank into the female die without substantial stretch orientation. The method is currently in use for forming relatively thin-walled articles e.g., from sheets of as little as 0.25 mm, or greater, thickness, whereas stretch forming is particularly adapted for forming thicker-walled articles, e.g., from sheets or billets of as much as 20 mm, or greater, thickness. In the pressure-forming method the thickness of the side walls is a function of the thickness of the original blank and of the wall area of the formed article.
At times it may be desirable to provide by stretch forming or pressure forming articles having greater wall thickness without increasing the thickness of the billet from which the article is formed.